This invention relates to high resolution multicolor ink jet printers and, more particularly, to a high resolution printer providing continuous tone color image characteristics.
In many instances, as for example in proofing systems for digital color pre-press operations, it is important to verify the integrity of digitally created color images prior to the production of film or plate images to assure the faithfulness of the image to be reproduced in the printed product. While such pre-proofing systems have been utilized previously with other printing techniques, the provision of an ink jet pre-proofing system has unique advantages in processing simplicity, high resolution and digital image control.
In high resolution ink jet systems i.e., those having about 235 or more dots/cm, drop placement errors which degrade image quality can be produced in many ways. For example, the position of an individual ink drop projected from a selected ink jet orifice in the printhead with respect to the intended location of the ink drop may be subject to errors in either the main scanning of the subscanning direction resulting from misplacement of the head itself or an incorrect angular orientation of the arrays of orifices in the printhead, or from variations in the spacing between the ink jet head and the substrate toward which the ink drops are projected. The effect of such errors on the visual appearance of a printed image depends upon the spacing of the drop from adjacent ink drops in the image and the density and color differences between the adjacent drops or image segments. For high quality images the result of such errors should be below the limit of visual detectability.
Ink jet systems have the disadvantage that variations in tone, or density level, of an image pixel, which are effected in the graphic arts by varying the physical size of each image element, are difficult to achieve in the same manner. Although it is possible, as described for example in the Sakurada et. al. U.S. Pat. No. 4,672,432 and the Kouzato U.S. Pat. No. 4,686,538, to vary the effective area of each pixel by varying number of ink jet dots provided in a matrix corresponding to the image pixel and thereby vary the pixel density, for high resolution systems such arrangements would require extremely small drop size and complex drop positioning control systems in order to achieve the desired result. Similarly, arrangements for controlling pixel density by varying the overlap of adjacent dots produced by ink jet drops, as described, for example, in the Saito et. al. U.S. Pat. No. 4,692,773 involve complex selective drop placement techniques. For multicolor images, moreover, two or more subtractive color ink drops must be precisely positioned at the same location in order to provide the desired hue.